If you're a developer who's asked to put in a backdoor to your encryption, you have the opportunity to do a very funny thing by putting one in that doesn't work properly. If they wanted it done well, they should have done it themselves. Make it so that all messages decrypt to a copypasta, or that the key is wrong so they all decrypt to nonsense. Make it so that trying to decrypt a message encrypts it further. Tell them the encryption is ROT26. Make it only work sometimes. Encryption is a black box. Use that.